The present invention relates generally to devices for measuring reflective, transmissive, or self-luminous samples and reporting their spectrophotometric, spectroradiometric, densitometric, or other colorimetric appearance attributes.
Users of desktop color systems have a need to accurately measure color. Color systems end-users expect accurate color matching between their source (scanner or monitor) and the color hardcopy produced by their color digital printer. In order to achieve WYSIWYG ("What You See Is What You Get") color, color imaging devices must be characterized and calibrated transparently to the user, and work seamlessly with popular software applications. Support for import and export of image data described in a device independent format such as CIE is increasingly a requirement.
The inputting of colors represented by physical samples into an electronic design for display and printing is currently a tedious process. The end-user has several options. The sample may be scanned, but the color reproduction will be poor for the reasons discussed above. The sample may be visually matched by specifying RGB or CMYK amounts for the application, but the match will be device dependent and highly variable. A color specification system may be employed by visually matching the sample to one of the specified colors then entering the color specification to the application, but the match will be device dependent, highly variable, and rely upon application compatibility with the color specification system.
One application where WYSIWYG is particularly important is color desktop publishing. In many desktop and workstation environments, color end-users (e.g. publishing, prepress, design, graphics, etc.) have a wide selection of input and output devices (e.g. scanners, monitors, printers, imagesetters, etc.) and color creation applications. Since this environment is generally open-architecture, the calorimetric characteristics of the various devices and applications are not, and cannot be, well-matched due to the multi-vendor nature of the market. As a result, the quality of color reproduction between input and output is highly variable and generally poor. In order to achieve consistent color matching in this highly disparate environment, several companies have introduced software-based color management system (CMS) technology based on device independent color. The principle of operation of such systems is to reference all devices (device dependent) to a common CIE color space (device independent). A simple work flow for such a CMS might be comprised of the following steps: (1) the image scan (Scanner RGB) is referenced to CIE; (2) the image is converted to Display RGB for editing; (3) the displayed image is referenced back to CIE; and (4) the image is converted to printer CMYK for output.
Generally, the CMS vendor provides several key elements: a library of device characterization profiles, a color matching method, and color transformation software. Since the expertise and equipment required for the creation of profiles is expensive, device characterization is performed by the CMS vendor for an average device for average viewing conditions. Unfortunately, characterization accomplished in this manner is done under factory conditions and not for the end-user conditions (device, light source, media, viewing, etc.). Since an end-user's device will be quite different from the device that was profiled at the factory, some vendors offer hardware/software calibration. Calibration only partially compensates for the differences between factory conditions and end-user conditions but does not satisfy the need for end-user device characterization compatible with open architecture CMS. However, there are no end-user characterizors for ambient, display, or hardcopy characterization due to the high cost of such equipment and lack of suitable end-user software. Currently, CMS products are generally proprietary and must be purchased from the CMS vendor. As open architecture CMS technology will be included as part of the operating system, device manufacturers, third parties, and end-users will require custom CMMs or profiles.
The combination of CMS technology, factory characterization, and end-user calibration improves the average color reproduction on the desktop somewhat, but is expensive, complex, and does not meet the end-user requirements for quality, cost, speed, and compatibility. End-users need a fast, low-cost, simple hardware/software characterizor enabling them to easily characterize (not just calibrate) their specific scanners, monitors, and printers to their specific viewing conditions in a manner that is compatible with open architecture CMS technology.
U.S. Pat. No. 5,137,364 discloses a color sensor that employs a plurality of LEDs (light emitting diodes) and an array of photodetectors. The McCarthy sensor utilizes individually addressable, customized LEDs as its illumination sources and a photodetector array, where each photodetector measures reflectance of the sample at a different visible wavelength, and where each element's spectral sensitivity must be individually optimized. The McCarthy sensor also utilizes a single beam reflectance measurement scheme. Thus, the photodetectors must be made as stable as possible by maintaining at a constant temperature and being protected from humidity, etc. Furthermore, the McCarthy device requires expensive and stable components since the measured energy collected from the color sample must be constant. Such expensive and customized components nevertheless may also experience long term drift and may be highly sensitive to noise.
U.S. Pat. No. 5,377,000 to Berends discloses a color sensor that utilizes a single illumination source positioned directly above the sample, 21 sample photodetectors arranged circumferentially around the illumination source and positioned at 45.degree. angles with respect to the sample, and 2 reference photodetectors positioned to receive light directed from the single illumination source. Berends utilizes a "pseudo-dual beam" reflectance measurement scheme in an attempt to eliminate the need of an additional 19 reference photodetectors that would be required in a classic dual-beam reflectance measurement scheme. This is performed by configuring the 2 reference photodetectors to sample the illumination source at opposite extremes of the visible light spectrum, and by applying a "least squares fit" calibration calculation to simulate the required 21 reference channel readings.